Method for hot working

ABSTRACT

A practice for hot working, as by rolling, ingot castings of cobalt- and nickel-base alloys to avoid cracking thereof; prior to working the casting is surrounded with an envelope of mild steel to form an assembly that is heated to an elevated temperature and subjected to fluid-pressure compacting to bond the envelope to the casting; thereafter, the assembly is hot worked to the desired mill product form, and at the completion of hot working the envelope is removed.

United States Patent [191 Ross et a1.

[ METHOD FOR HOT WORKING [75] Inventors: Stuart T. Ross, Manlius; Brian R.

Leslie, Camillus, both of NY.

[73] Assignee: Crucible Inc., Pittsburgh, Pa.

22 Filed: Jan. 24, 1973 [21] Appl. No.: 326,341

[52] US. Cl 29/423, 29/481, 29/494, 29/4975 [51] Int. Cl B23p 17/00 [58] Field of Search 29/480, 481, 494, 4975, 29/423, 424; 425/78, 405 H [56] References Cited UNITED STATES PATENTS 2,874,453 2/1959 Losco et a1. 29/4975 X 3,122,423 2/1964 Hessler 29/1875 3.286.337 11/1966 Sauve 29/423 3,339,271 9/1967 Durfee et 211... H

[ Mar. 26, 1974 3,561,099 2/1971 Mizukara 29/480 X 3,571,850 3/1971 Pohto 425/405 H 3.604,102 9/1971 Boccalari 29/481 X FOREIGN PATENTS OR APPLICATIONS 1,358,464 2/1963 France 29/480 Primary ExaminerJ. Spencer Overholser Assistant Examiner-Ronald .1. Shore [57] ABSTRACT A practice for hot working, as by rolling. ingot castings of cobaltand nickel-base alloys to avoid cracking thereof; prior to working the casting is surrounded with an envelope of mild steel to form an assembly that is heated to an elevated temperature and subjected to fluid-pressure compacting to bond the envelope to the casting; thereafter, the assembly is hot worked to the desired mill product form, and at the completion of hot working the envelope is removed.

7 Claims, 6 Drawing Figures FATENTED MAR26 1914 SHEET 1 (1F 2 PATENTEDMARZS I974 3 798' 742 sum 2 or 2 F/GI 6 METHOD FOR HOT WORKING Various superalloys that are supplied in the form of billets and bars are processed by hot-working, which includes forging and hot-rolling to the desired mill product form. Cobaltand nickel-base alloys of this type which are of an alloy composition that provides for matrix strengthening, rather than dispersion strengthening, to achieve high strength at elevated temperatures exhibit an extremely narrow hot-working range. Matrix strengthened alloys of this type and exhibiting a narrow hot-working range include S-816, I-IASTELLOY, Inconel 718 and Rent: 41 and 95. These alloys have a hot working range of 200 F or less. If these alloys are processed above the hot-working limit, the matrix strengthening elements will be taken into solution and thereafter during cooling form intermetallic compounds rather than remain present in the matrix to achieve the desired strengthening effect. On the other hand, and more importantly, if these alloys are hotworked at a temperature below the hot-working range, cracking during hot working, and particularly hot rolling, will result. Cracking is particularly prevalent at the corners of the workpiece during working, because these areas cool preferentially to the remainder of the workpiece.

It is, therefore, an object of the present invention to provide a practice for hot working castings of alloys of this type that avoids cracking during working if the temperature falls below the customary hot-working range.

These and other objects of the invention, as well as a complete understanding thereof, may be obtained from the following description, specific examples and drawings, in which:

FIG. 1 is a schematic showing of a cross-section of an ingot positioned within an envelope for hot-working in the conventional manner;

FIG. 2 is a schematic showing of a cross-section of the ingot and envelope of FIG. 1 after hot working;

FIG. 3 is a schematic showing of a cross-section of an ingot with an envelope bonded thereto by isostatic compacting in accordance with this invention and prior to hot working;

FIG. 4 is a schematic showing of a cross-section of the ingot and envelope of FIG. 3 after hot-working;

FIG. 5 is a photograph of a cross-section of an ingot and envelope bonded in accordance with the invention and prior to hot working; and

FIG. 6 is a photograph ofa cross-section of the ingot and envelope of FIG. 5 after hot working.

Broadly, in the practice of the invention, castings, which may typically be ingot castings, of cobaltand nickel-base alloys are enclosed in an envelope of an iron-base alloy, which may typically be mild steel. The resulting assembly is heated to an elevated temperature at which hot working is to be effected. Upon reaching elevated temperature the assembly is isostatically compacted at a combination of pressure and temperature to produce a bond between the envelope and the casting. The assembly is then hot worked at elevated temperature. Customarily after the completion of hot working the envelope is removed from the assembly. This may be achieved by the use of conventional conditioning techniques, such as grinding or a combination of grinding and acid pickling.

Preferably, the envelope is provided with a stem or other means for connecting the interior of the same to an evacuation means such as a pump; during the early stages of heating to compacting temperature the interior of the envelope is evacuated to remove gasification products, which principally are oxides which have volatilized during heating. In this manner, a relatively oxide-free casting and envelope surface are provided to facilitate bonding during subsequent isostatic compacting. After evacuation, and prior to compacting, the envelope is sealed against the atmosphere.

After the assembly has been subjected to hot isostatic compacting to produce a bond between the envelope and the casting, hot working of the assembly will include forging followed by at least one but generally a plurality of hot-rolling operations.

Isostatic compacting to achieve a bond may be effected by the use of any fluid-pressure compacting apparatus; however, it is preferred to use a gas-pressure vessel, commonly termed an autoclave, which may be of the type disclosed in Boyer U.S. Pat. No. 3,543,345.

For alloys of the type to which the invention is particularly directly, which are matrix strengthened alloys of cobalt and nickel, compacting temperatures within the range of l,800 to 2,500" F at compacting pressures of 10,000 to 20,000 psi will be suitable to achieve the required bond between the envelope and the casting. It is to be understood, however, that the invention has utility in any hot-working application wherein the customary permissible hot-working temperature range is narrow.

To demonstrate the invention by way ofa specific example a 12% inches ingot of the following conventional WF-ll composition was processed:

C0 Balance Ni l0.0

C Mn Si Cr 0.l5 l.500.50 20.0

The ingot was first conditioned to remove surface oxide scale and then placed in a mild steel tube having an interior diameter of 12 inches and a wall thickness of one-fourth inch. A bottom plate of mild steel with a thickness of one-fourth inch was welded to close one end of the tube, and after insertion of the ingot into the tube a top was welded onto the open end of the tube. This top was provided with a stem suitable for connection to a vacuum pump. The assembly consisting of the ingot and envelope was placed in a furnace and heated to a temperature of about 2,200 F; during the early stages of heating the interior of the mild steel envelope was connected to a vacuum pump via the stem and the gaseous reaction products were removed. Thereafter the stem was closed to seal the interior of the envelope. The assembly, upon reaching a temperature of 2,200 F, was transferred to an autoclave and while at a temperature of about 2,l F was compacted by the use of nitrogen gas at a pressure of about 17,000 psi. After compacting the assembly was heated to a temperature of about 2,I75 F, which was the desired hot-working temperature. It was then forged to the form of an 8 in. square. This forging was then hot rolled to a 3-% in. square. Thereafter the assembly was again reheated to a temperature of about 2,l75 F and rolled to a 1-5/16 in. square. Upon cooling the envelope was removed by a grinding operation. The envelope was uniformly bonded over the entire workpiece surface at a uniform thickness of about one-sixteenth in. Removal of the envelope resulted in only a 6 percent yield loss. Typically yield losses during processing of this type material in a conventional manner may be as high as 25 percent.

It may be seen from the above that only two hotrolling operations were necessary to reduce the ingot to the desired mill product form. Typically, five separate hot-rolling operations with intermediate conditioning and reheating are required. It is understood, of course, that each conditioning operation increases the yield loss. With the practice of the invention, however, the envelope retards heat loss and also serves to support the corners of the workpiece during hot-working, which enables a uniform distribution of the hotworking temperatures to prevent cracking. Therefore, greater reductions may be effected during each hotrolling operation, and since the workpiece is enclosed the problem of surface scale formation during hot working is avoided, thereby avoiding intermediate conditioning operations which result in yield loss.

It is critical in the practice of the invention that prior to any hot working, such as by forging or hot rolling, that a bond be achieved between the envelope and the casting. This bond may be either mechanical or metallurgical or a combination of both; however, under typical fluid-pressure compacting practice in accordance with the invention a mechanical bond will be achieved with some degree of metallurgical bonding resulting primarily from diffusion of carbon from the mild steel envelope to the casting. If a bond is not provided during the compacting operation and prior to hot working the assembly will not elongate unitarily during hot working. This results in processing difficulties, such as the container being folded or buckled into the casting so that upon the completion of hot rolling the envelope is not of a substantially uniform thickness over the workpiece surface. Therefore, during conditioning to remove the envelope, as by surface grinding, irregular surface portions must be removed to effect complete removal of the envelope, which obviously results in lower yield and poor product quality.

With reference to the drawings, FIG. 1 thereof shows in cross-section an ingot l enclosed in an envelope 12 in preparation for hot working. FIG. 2 shows schematically the typical cross-section of the assembly of FIG. 1 after hot rolling. The envelope has not elongated uniformly with the ingot but instead has been folded and buckled by the rolling forces into the ingot body in an irregular fashion. Consequently, during removal of the envelope, as by a grinding operation, high yield loss will result. In contrast, FIG. 3 shows an assembly consisting of an ingot l4 and an envelope 16 with the envelope bonded to the ingot, which bond is designated as 18, by hot isostatic compacting in accordance with the invention. As may be seen from the schematic showing of the cross-section of the assembly of FIG. 3 after hot rolling, because of the bond between the ingot and envelope they elongated unitarily during hot rolling to result in the envelope being at a relatively uniform thickness over the ingot surface. The effect may also be observed in FIGS. 5 and 6. FIG. 5 is a photograph of a partial cross-section of an ingot of WF- 1 l alloy that has been hot isostatically bonded to a mild steel envelope in accordance with the practice of the invention. The bond produced between the envelope and ingot as a result of hot isostatic compacting may be readily observed in this photograph. FIG. 6 is a photograph of a crosssection of the assembly of FIG. 5 after hot-rolling. As may be seen from this photograph, the envelope elongated with the ingot relatively uniformly during hot rolling.

We claim:

1. A method for hot working castings of cobaltand nickel-base alloys, comprising enclosing a cobaltor nickel-base alloy casting to be hot worked within an envelope of an iron-base alloy to form an assembly, heating said assembly to an elevated temperature, isostatically compacting said assembly with a combination of pressure and temperature to produce a bond between said envelope and said casting and hot working said assembly to achieve a reduction in a cross-sectional area thereof.

2. The method of claim I wherein said envelope is removed from said assembly after the same has been hot worked.

3. The method of claim 1 wherein after said casting is enclosed in said envelope, the interior of said envelope is evacuated during said heating and is sealed against the atmosphere prior to said compacting.

4. The method of claim 1 wherein said hot working of said assembly includes forging followed by at least one hot-rolling operation.

5. The method of claim 1 wherein said compacting is performed in a gas pressure vessel.

6. The method of claim 5 wherein said compacting is at a pressure within the range of 10,000 to 20,000 psi and at an assembly temperature within the range of 1,800 to 2,500 F.

7. A method for hot working castings of cobaltand nickel-base alloys, comprising enclosing a cobaltor nickel-base alloy casting to be hot worked within an envelope of an iron-base alloy to form an assembly, heating said assembly to a temperature within the range of 1,800 to 2,500 F, evacuating said envelope during said heating, sealing said envelope against the atmosphere, isostatically compacting said assembly in a gas pressure vessel at a pressure sufficient at the assembly temperature to produce a bond between said envelope and said casting, hot working said assembly to achieve a reduction in a cross-sectional area thereof and removing said envelope from said assembly after the same has been hot worked.

i t t i i 

1. A method for hot working castings of cobalt- and nickel-base alloys, comprising enclosing a cobalt- or nickel-base alloy casting to be hot worked within an envelope of an iron-base alloy to form an assembly, heating said assembly to an elevated temperature, isostatically compacting said assembly with a combination of pressure and temperature to produce a bond between said envelope and said casting and hot working said assembly to achieve a reduction in a cross-sectional area thereof.
 2. The method of claim 1 wherein said envelope is removed from said assembly after the same has been hot worked.
 3. The method of claim 1 wherein after said casting is enclosed in said envelope, the interior of said envelope is evacuated during said heating and is sealed against the atmosphere prior to said compacting.
 4. The method of claim 1 wherein said hot working of said assembly includes forging followed by at least one hot-rolling operation.
 5. The method of claim 1 wherein said compacting is performed in a gas pressure vessel.
 6. The method of claim 5 wherein said compacting is at a pressure within the range of 10,000 to 20,000 psi and at an assembly temperature within the range of 1,800* to 2,500* F.
 7. A method for hot working castings of cobalt- and nickel-base alloys, comprising enclosing a cobalt- or nickel-base alloy casting to be hot worked within an envelope of an iron-base alloy to form an assembly, heating said assembly to a temperature within the range of 1,800* to 2,500* F, evacuating said envelope during said heating, sealing said envelope against the atmosphere, isostatically compacting said assembly in a gas pressure vessel at a pressure sufficient at the assembly temperature to produce a bond between said envelope and said casting, hot working said assembly to achieve a reduction in a cross-sectional area thereof and removing said envelope from said assembly after the same has been hot worked. 